Constructors in JavaScript objects

Question

Can JavaScript classes/objects have constructors? How are they created?

Answer 1

Yes, you can define a constructor inside a class declaration like this:

class Rectangle {
  constructor(height, width) {
    this.height = height;
    this.width = width;
  }
}

Answer 2

Using Nick's sample above, you can create a constructor for objects without parameters using a return statement as the last statement in your object definition. Return your constructor function as below and it will run the code in __construct each time you create the object:

function Box()
{
   var __construct = function() {
       alert("Object Created.");
       this.color = 'green';
   }

  this.color = '';

   this.getColor = function() {
       return this.color;
   }

   __construct();
}

var b = new Box();

Answer 3

Example here: http://jsfiddle.net/FZ5nC/

Try this template:

<script>
//============================================================
// Register Namespace
//------------------------------------------------------------
var Name = Name||{};
Name.Space = Name.Space||{};

//============================================================
// Constructor - MUST BE AT TOP OF FILE
//------------------------------------------------------------
Name.Space.ClassName = function Name_Space_ClassName(){}

//============================================================
// Member Functions & Variables
//------------------------------------------------------------
Name.Space.ClassName.prototype = {
  v1: null
 ,v2: null
 ,f1: function Name_Space_ClassName_f1(){}
}

//============================================================
// Static Variables
//------------------------------------------------------------
Name.Space.ClassName.staticVar = 0;

//============================================================
// Static Functions
//------------------------------------------------------------
Name.Space.ClassName.staticFunc = function Name_Space_ClassName_staticFunc(){
}
</script>

You must adjust your namespace if you are defining a static class:

<script>
//============================================================
// Register Namespace
//------------------------------------------------------------
var Shape = Shape||{};
Shape.Rectangle = Shape.Rectangle||{};
// In previous example, Rectangle was defined in the constructor.
</script>

Example class:

<script>
//============================================================
// Register Namespace
//------------------------------------------------------------
var Shape = Shape||{};

//============================================================
// Constructor - MUST BE AT TOP OF FILE
//------------------------------------------------------------
Shape.Rectangle = function Shape_Rectangle(width, height, color){
    this.Width = width;
    this.Height = height;
    this.Color = color;
}

//============================================================
// Member Functions & Variables
//------------------------------------------------------------
Shape.Rectangle.prototype = {
  Width: null
 ,Height: null
 ,Color: null
 ,Draw: function Shape_Rectangle_Draw(canvasId, x, y){
    var canvas = document.getElementById(canvasId);
    var context = canvas.getContext("2d");
    context.fillStyle = this.Color;
    context.fillRect(x, y, this.Width, this.Height);
 }
}

//============================================================
// Static Variables
//------------------------------------------------------------
Shape.Rectangle.Sides = 4;

//============================================================
// Static Functions
//------------------------------------------------------------
Shape.Rectangle.CreateSmallBlue = function Shape_Rectangle_CreateSmallBlue(){
    return new Shape.Rectangle(5,8,'#0000ff');
}
Shape.Rectangle.CreateBigRed = function Shape_Rectangle_CreateBigRed(){
    return new Shape.Rectangle(50,25,'#ff0000');
}
</script>

Example instantiation:

<canvas id="painting" width="500" height="500"></canvas>
<script>
alert("A rectangle has "+Shape.Rectangle.Sides+" sides.");

var r1 = new Shape.Rectangle(16, 12, "#aa22cc");
r1.Draw("painting",0, 20);

var r2 = Shape.Rectangle.CreateSmallBlue();
r2.Draw("painting", 0, 0);

Shape.Rectangle.CreateBigRed().Draw("painting", 10, 0);
</script>

Notice functions are defined as A.B = function A_B(). This is to make your script easier to debug. Open Chrome's Inspect Element panel, run this script, and expand the debug backtrace:

<script>
//============================================================
// Register Namespace
//------------------------------------------------------------
var Fail = Fail||{};

//============================================================
// Static Functions
//------------------------------------------------------------
Fail.Test = function Fail_Test(){
    A.Func.That.Does.Not.Exist();
}

Fail.Test();
</script>

Answer 4

This is a constructor:

function MyClass() {}

When you do

var myObj = new MyClass();

MyClass is executed, and a new object is returned of that class.

Answer 5

I guess I'll post what I do with javascript closure since no one is using closure yet.

var user = function(id) {
  // private properties & methods goes here.
  var someValue;
  function doSomething(data) {
    someValue = data;
  };

  // constructor goes here.
  if (!id) return null;

  // public properties & methods goes here.
  return {
    id: id,
    method: function(params) {
      doSomething(params);
    }
  };
};

Comments and suggestions to this solution are welcome. :)

Answer 6

In JavaScript the invocation type defines the behaviour of the function:

• Direct invocation func()
• Method invocation on an object obj.func()
• Constructor invocation new func()
• Indirect invocation func.call() or func.apply()

---

The function is invoked as a constructor when calling using new operator:

function Cat(name) {
   this.name = name;
}
Cat.prototype.getName = function() {
   return this.name;
}

var myCat = new Cat('Sweet'); // Cat function invoked as a constructor

Any instance or prototype object in JavaScript have a property constructor, which refers to the constructor function.

Cat.prototype.constructor === Cat // => true
myCat.constructor         === Cat // => true

Check this post about constructor property.